Pointless whataboutery fallacy. Of course you should tip. The fact the society seems we tip one group of people and not others isn’t relevant to the question ‘should you tip for pizza’.
So do you offer tips to everybody who ever serves you in any way? I know not all will be allowed or want to accept them, but some would. If not, why do you feel so strongly about takeaway delivery people but not others on low wages in service industries? Society will only change if there's a groundswell of people who believe that arbitrary tipping gives low-paid workers more assurance than proper regular wages, and put their money where their mouth is - every single time.
If it's accepted that some jobs are routinely artificially propped up by customers making up the wages to something more realistic, nothing will ever change. I'm not talking about an occasional gift of gratitude for outstanding service or a token present of appreciation at Christmas - I mean the deliberate outsourcing of a big proportion of a fair wage to the whims and guilt feelings of customers.
It seems counter-productive to me and actually quite patronising to younger/lower-paid workers to accept that their employment should by custom be subject to permanent uncertainty. You would never find an accountant or plumber willing to issue artificially low invoices to customers and hinting that they'd like them to pay extra, if they think the person did their job well - and impressing on them that they won't be able to feed their families or pay their electricity bill should the customer decline to do so.
If heavy tipping subsidies stopped tomorrow, a lot of workers just wouldn't be in a position to keep working in these jobs - and then the employers would be forced either to go out of business or to start implementing a fairer economic model for everybody - workers knowing that they will be guaranteed a fair wage and customers knowing that the price quoted is indeed the actual price they're expected to pay.
We do it with Fairtrade for farmers and producers in developing countries - agree an acceptable price to pay them for their hard work and not just pay them rock bottom, figuring that 'well, otherwise they'd get nothing at all' - and maybe put the farmer's bank details on the pack of tea bags, in case any end consumers in wealthy countries happen to feel particularly generous and 'toss a pound or two to the poor person'. I'm sure that would help the farmer plan their livelihood confidently and give them an assured fair level of regular income 